There's currently a limitation where mmhmm cannot pick up screens it cannot "see". You will not be able to create a screen share slide in cases where the window to be shared:
- is minimized
- is on a different desktop or monitor
- otherwise invisible to mmhmm, possibly because mmhmm is in full-screen mode
To resolve this issue, please make sure mmhmm is not full-screened and that the window you want to share is open (i.e. not minimized) on the same desktop. You should then be able to share that window to a slide.